Link adaptation by means of adaptive modulation and coding enables robust and spectrally efficient transmission over time-varying channels in a wireless system. The basic idea behind it is to estimate the channel at a receiver and feed this estimate back to a transmitter. The latter adjusts its transmission in order to adapt the modulation scheme and the code rate to the channel characteristics. Modulation and coding schemes that do not adapt to fading conditions require a fixed link margin to support an acceptable performance when the channel quality is poor. These systems are then designed for the worst-case channel conditions, and result in an inefficient use of the channel. Thus, adaptive modulation and coding schemes are appealing, since they can increase the average throughput, and reduce the required transmit power and bit error rate. See, for example: “Variable-Rate Transmission for Rayleigh Fading Channels” by J. K. Cavers (IEEE Transactions on Communications, pp. 15-22, February 1972); “Variable Rate QAM for Mobile Radio” by W. T. Webb and R. Steele (IEEE Transactions on Communications, pp. 2223-2230, July 1995); “An Adaptive Coding Scheme for Time-Varying Channels” by B. Vucetic (IEEE Transactions on Communications, pp. 653-663, May 1991); and “Adaptive-Modulation Schemes for Minimum Outage Probability in Wireless Systems” by K. M. Kamath and D. L. Goeckel (IEEE Transactions on Communications, pp. 1632-1635, October 2004).
Moreover, a radio channel is always subject to some degree of frequency selectivity, implying that the channel quality will vary in the frequency domain. This variation in frequency can be beneficial for a link adaptation scheme over the frequency axis for multi-carrier systems such as OFDM systems. With adaptive modulation and coding in the frequency domain, a higher-order modulation (e.g. 16QAM or 64QAM) together with a high code rate is appropriate for frequency intervals (e.g., subcarriers or groups of sub-carriers) experiencing advantageous channel conditions in the frequency domain, where QPSK modulation and low-rate coding are used for frequency intervals with poor radio link conditions.
The advantages of adaptive modulation and coding have motivated its use in advanced wireless communication systems, including cellular systems like EGPRS and HSPA as well as wireless LANs.
FIG. 1 is a schematic diagram of a receiver 2 communicating with a transmitter 4 via a wireless channel 6. The transmitter and receiver can be a base station (or Node-B) and a mobile terminal (or user equipment (UE)), adapted for use in a wireless cellular environment. The receiver 2 has a processor 8 for, amongst other things, estimating the channel quality and providing a channel quality indicator (CQI) value. The transmitter includes a processor 10 for, amongst other things, implementing adaptive modulation and coding based on CQIs it receives from the receiver. It will readily be appreciated that FIG. 1 is greatly simplified—in practice there can be a plurality of receivers and transmitters, with a multiplicity of channels, which vary in quality with time and in the frequency domain.
Another appealing scheme that maximizes the spectral efficiency of a wireless system is channel-dependent scheduling implemented by a packet scheduler 14. This mechanism controls the allocation of the shared resources (e.g. frequency intervals for multi-carrier system like OFDM) at each instant. It is closely related to adaptive modulation and coding scheme and often they are seen as one joint function as they are both trying to adapt to the channel conditions. The first objective of downlink scheduling, for example, is to make the users orthogonal by different multiplexing techniques: Time Division Multiplexing, Frequency Division Multiplexing, Code Division Multiplexing or Spatial Division Multiplexing. The second objective is the maximization of the radio resources. When transmissions to multiple users occur in parallel, resources (in code/frequency/time/space domain) are assigned to users with the best instantaneous channel conditions. See, for example “Information Capacity and Power Control in Single-Cell Multiuser Communications” by R. Knopp and P. A. Humblet (Proceedings of IEEE International Conference on Communications, vol. 1, Seattle, USA, June 1995, pp. 331-335). This strategy is an example of channel dependent opportunistic scheduling, where the scheduler takes into account only the instantaneous radio-link conditions. Other strategies can take into account delay, fairness—in general parameters related to QoS constraints.
To enable adaptive modulation and coding and channel-dependent opportunistic scheduling, the mobile terminal or User Equipment (UE) reports a Channel Quality Indicator (CQI) 12. In an OFDM system, to support downlink scheduling in the frequency domain coupled with a link adaptation scheme with a plurality of users, each user needs to report a CQI per frequency interval (group of sub-carriers) and over time. In order to provide full flexibility at the packet scheduler 14, the UE 2 would need to report CQIs 12 over the entire frequency band, making the signaling overhead impractically large.
FIG. 2 is a schematic diagram of a frequency band with consecutive frequency intervals f1, . . . , fN at respective frequency locations i=1, . . . , N. Each frequency interval corresponds for instance to a predefined number of adjacent OFDM sub-carriers.
Many solutions have been proposed for CQI feedback reduction, to allow frequency selective scheduling with reduced signaling overhead. These solutions range from uniform reporting grid of CQI, where a CQI (CQI(1), . . . , CQI(N) is reported for each frequency interval f1, . . . , fN (impractical from the point of view of the implementation complexity), to reporting only the indication of the best CQI and the best frequency interval (e.g. CQI(i) for interval fi). The latter scheme is based on the fact that a user will be preferentially scheduled on its best frequency interval, and therefore the CQI for the best resource block is of primary interest to the scheduler 14. A variation of the latter scheme is to report the CQIs for the M best frequency intervals, e.g. CQI(1), CQI(j), CQI(i+k) for f1, fi and fi+k (M=3). Another possibility is to adopt a threshold-based CQI report, based on the feedback of the average CQI over the frequency intervals that are within a predefined threshold from the best CQI. Other solutions aim at reducing the overhead of CQI feedback by reporting an average CQI over the M best frequency intervals, or over the frequency intervals that are within a predefined threshold from the best CQI. On top of these schemes, it has been proposed to apply some known compression methods such as Discrete Cosine Transform or Wavelet Transform to further reduce the number of bits required to encode the CQI reports.
The main disadvantages of these schemes are:                Lack of scheduling flexibility—For different reasons such as system overload, scheduling type or practical reasons, the packet scheduler may need to assign to the users different frequency intervals than the one reported. The CQI schemes reported above do not provide channel quality information for other than the reported frequency intervals.        In the case of feedback of averaged CQIs, reporting averaged channel quality may destroy channel information. Depending on channel variability in frequency and/or time, the averaging could destroy the information about the channel condition at specific frequencies.        Signaling overhead—The schemes mentioned above need to report the index i of frequency intervals to which the reported CQIs are referred.        